1. Field of the Invention
The present invention relates to a projector using liquid crystal cells, and more particularly to a color projector with an arrangement of polarizers and liquid crystal cells which are aligned on an axis of light to be projected.
2. Description of the Related Art
Conventionally, a color projector had three black and white cathode-ray tubes (CRTs). Each CRT reproduces an image for one primary color. The images reproduced by the three CRTs are colored by dichroic mirrors in three primary colors, respectively. Thereafter they are projected on a screen through a projection lens to realize the full-color reproduction of an image. In such a conventional apparatus, however, the CRT did not have the sufficient brightness for projection on a large-sized screen and in addition, the size of the apparatus was considerably large, because three CRTs were used. Also, the apparatus was very expensive.
In order to make an apparatus small in its size and low in its price, various kinds of projectors using liquid crystal cells have been proposed. As shown in FIG. 1a, for example, a typical one thereof uses a neutral polarizer 2 and three Guest-Host type liquid crystal cells 4, 6, 8, which are colored in cyan, magenta and yellow, respectively. They are aligned on an axis of light to be projected. In this example, as apparent from the selection of color of the cells, the full-color reproduction was realized by a so-called subtractive mixture of color.
However, a colored image reproduced by this apparatus was low in the excitation purity of color and has no sufficient brightness. In addition, the scope of the color reproduction was narrow. These problems results mainly from the fact that the subtractive mixture of color was adopted for reproducing a color image.
Another example of the proposed projectors is, as shown in FIG. 1b, one using three colored pleochroic polarizers 10, 14, 18, three twisted nematic liquid crystal cells 12, 16, 20 and a netural polarizer 22. The three pleochroic polarizers are colored in red, green and blue which are three primary colors in a so-called additive mixture of color.
According to this example, the brightness of a reproduced image is improved because of the adoption of the additive mixture of color. At present, however, it is very difficult to get, at a low price, colored pleochroic polarizers for all the three colors, which have necessary color characteristics, such as dichroic ratio, brightness etc.. If polarizers having excellent color characteristics are used for the three colors in order to obtain a reproduction of an image with the high excitation purity and the sufficient brightness, an apparatus will become considerably expensive. In addition, although the primary colors of red, green and blue can be easily reproduced, the driving control of liquid crystal cells becomes somewhat complicated when cyan, magenta and yellow are reproduced.